Marine waters cover 70.8% of the earth’s surface. And, if we substract the 5.5% area of the continental shelf, 65.3% of the earth’s surface is occupied by continental slope and rise, and deep-sea parts (data after Dietrich and Kalle 1957). The deep sea can be subdivided into several topographical units. There are deep-sea grabens (15 in number), which cut deeper than 6,000 m. Deep-sea grabens are restricted to tectonic active regions near the foot of continental slopes.